Bivalent OPV
In the remaining four polio endemic countries - Nigeria, India, Pakistan and Afghanistan - there has been co-circulation of wild poliovirus types-1 and 3 in some infected areas. This co-circulation has resulted in calls for the tests and potential licensing of a bivalent oral poliovirus vaccine (bOPV) for use in difficult areas where type-1 and 3 viruses remain endemic.
Proponents argue that a bivalent vaccine would improve overall immunity levels by combining type 1 and 3 vaccines into a single dose. Hesitation by regulatory agencies to support licensure of the bivalent vaccine stems from the lack of efficacy and safety data that were commonly available for monovalent OPV trials. Of particular importance is the degree to which a bOPV would compromise seroconversion to each serotype. In November 2007, the Advisory Committee on Poliomyelitis Eradication (ACPE) recommended conducting a trial to test the efficacy of OPV on seroconversion rates prior to pursuing vaccine licensure. To accomplish this, the group proposed the addition of a fourth arm to an mOPV seroconversion study in India planned for mid-2008.
Trivalent OPV (tOPV) has been in use globally since the 1960s, with greater than two billion doses having been administered annually. Monovalent OPV (mOPV) for each of the three poliovirus types were used during the development of tOPV. For a short time in 1959, bivalent OPV (bOPV) was experimented with using a balanced combination of mOPV1 and mOPV3 in Estonia, Hungary and Lithuania.
The immunogenicity of tOPV in tropical areas is variable and primarily stimulates seroconversion to type 2 virus and partial conversion to types 1 and 3. Until 2005, tOPV was the only oral vaccine licensed around the world and used by the GPEI.
In 2004, the GPEI embarked on an aggressive campaign to test and license the new monovalent vaccines. The primary goal was to boost per-dose immunity in Egypt, where low-level type 1 transmission persisted. This resulted in accelerated eradication efforts, since monovalent vaccines could be used to increase protection against the remaining two circulating wild virus serotypes. Monovalents can also be used to more effectively respond to vaccine- derived poliovirus (VDPV) outbreaks. In the post-eradication era, mOPVs can be used to respond to outbreaks of VDPVs by enhancing the response to the target and reduce the risk of causing non-target VDPVs.
To date, monovalent vaccines have shown to be three times as effective in inducing serologic responses as tOPV.
The development and use of mOPVs in national and sub-national campaigns has created the impetus to prioritize eradication of type-1 virus, which is more virulent and poses a greater threat to cross-border transmission as compared to type-3. Using a bOPV might impair the strategy of preferentially stopping type-1 virus first. Consequently, the ACPE stated that a bOPV would have to perform at least 80% as well as the monovalent OPVs for each strain to consider the effort of licensing.
Stay tuned for the results of this study in an upcoming issue of the Polio pipeline in mid–2009, to see if bOPV promises to be a useful tool in the armament of the eradication programme